In recent years, such monocrystals as are naturally present, which have a three-dimensional shape peculiar to the crystals, have been desired in various fields because of unknown properties thereof.
Examples of a method for producing an artificial corundum crystal include (1) the flame fusion method (Verneuil's technique) of dropping raw material powder of the corundum crystal into oxygen and hydrogen flame and simultaneously growing crystal grains; (2) the flux method of mixing raw material powder of the corundum crystal with an appropriate flux, melting the mixture in a crucible, and precipitating/growing the crystal while cooling the melted solution slowly, precipitating/growing the crystal while applying temperature gradient to the solution in the crucible, or precipitating/growing the crystal while vaporizing the flux; (3) the Czochralski method of melting raw material powder of the corundum crystal in a crucible, and pulling up the crystal from melt; and (4) a method of forming raw material powder of the corundum crystal into a shape, and then heating the shaped powder at a high temperature in a hydrogen gas atmosphere for a long time so as to sinter the powder.
In the flame fusion method (1), the growth rate of the crystal is large, so that the obtained crystal cannot be made into a high quality at ease. According to this method, a rodlike crystal is produced. Thus, at the time of actually using the crystal as a laser-oscillating material or the like, it is necessary to cut the produced rodlike crystal into a desired shape, and further the hardness of any artificial corundum crystal is high; accordingly, a problem that costs increase arises. While the artificial corundum crystal produced by this method contains no purities, natural corundum crystal contains impurities. Thus, they can easily be distinguished from each other. Consequently, the artificial corundum crystal has a drawback that it is very low in value as an ornament.
The Czochralski method (3) makes it possible to produce a crystal having a high purity. Accordingly, the crystal can be preferably used as a laser-oscillating material or the like. However, according to this method, a rodlike crystal is produced. Thus, at the time of putting the crystal into practical use, it is necessary to cut the rodlike crystal into a desired shape as described above, and further the hardness of any artificial corundum crystal is high; accordingly, a problem that costs increase arises. Furthermore, the artificial corundum crystal produced by this method has a high purity without containing any impurity, and is largely different from natural corundum crystal. Thus, the artificial corundum crystal has a drawback that the crystal is very low in value as an ornament. The Czochralski method is disclosed in, for example, the patent documents 1 and 2.
According to the method (4) of shaping followed by sintering, it is unavoidable to heat powder at high temperature for a long time. Thus, a large quantity of energy is needed, so as to cause a problem that costs increase. The method for the sintering is disclosed in, for example, the patent document 3.
It is known that according to the flux method (2), a tabular crystal can be obtained by using, as a flux, lithium oxide-lead oxide (fluoride), aluminum fluoride/sodium, lithium oxide-tungsten oxide-lead oxide (fluoride), bismuth oxide-lanthanum oxide-lead oxide (fluoride) or the like, and precipitating/growing a crystal while cooling the melted solution slowly. However, only a thin tabular crystal can be obtained. Thus, there arises a problem that costs increase when the crystal is put into practical use. The flux method is disclosed in, for example, the nonpatent literatures 1 and 2.
Among the corundum crystals, a dark red crystal to which chromium was added is generally called ruby. Since the amount of natural ruby produced is relatively low, there is need for a process for inexpensively producing an artificial corundum crystal near to natural ruby.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 7-277893
Patent Document 2: JP-A No. 6-199597
Patent Document 3: JP-A No. 7-187760
Nonpatent Literature 1: Elwell D., Man-Made Gemstones, Ellis Horwood Ltd., Chichester (1979)
Nonpatent Literature 2: Elwell D., Scheel H. J., Crystal growth from high-temperature solutions, Academic Press, London (1975)